1. Field of the Invention
The present invention relates to a power supply circuit for a video display device, and more particularly, to a power supply circuit capable of improving transient state characteristics of a TVS (Transient Voltage Suppressor) diode that is used in an inner snubber circuit unit. The present application is based on Korean Patent Application No. 2002-34977 filed on Jun. 21, 2002, which is incorporated herein by reference.
2. Description of the Prior Art
A video display device such as a television requires direct current (DC) power for driving inner components. For example, during a scanning with an up-and-down and left-to-right sweep of an electron beam projected from an electron beam gun, the DC power is used to deflect the electron beam projected from the electron beam gun. In order to obtain the DC power, a power supply device is used to smooth and rectify an alternating current (AC) received from an external source.
FIG. 1 is a circuit diagram showing a power supply device for a conventional video display device.
As shown in the circuit diagram, a power supply circuit comprises a rectifier circuit unit 10, a switching circuit unit 20, a power transformer 30, an output circuit unit 40, and a snubber circuit unit 50.
The rectifier circuit unit 10 rectifies an AC power externally inputted by using diodes D1xcx9c4 for a rectification and a capacitor C1. The voltage rectified by the rectifier circuit unit 10 is supplied to a primary coil of the power transformer 30. The power transformer 30 induces a voltage at a secondary coil by using an interaction occurring between the primary and secondary coils. At this time, the switching circuit unit 20 switches on/off the flow of the current along the primary coil of the power transformer 30, thereby controlling the voltage to be induced at the secondary coil of the power transformer 30. The output circuit unit 40 rectifies and smoothes the voltage induced at the secondary coil of the power transformer 30 to obtain a DC power.
The snubber circuit unit 50 consists of a residual current device (RCD) snubber circuit and first and second TVS (Transient Voltage Suppressor) diodes TVS1 and TVS2 serially connected to each other. When the switching circuit unit 20 switches on/off the current of the primary coil of the power transformer 30, the snubber circuit unit 50 absorbs and removes a transient voltage in a reverse direction that is generated at the power transformer 30. The first and second TVS diodes TVS1 and TVS2 are elements for protecting a weak circuit in an electrically transient state that is caused due to the electrostatic discharge, the inductive load switching or the induced lightning.
However, the first and second TVS diodes TVS1 and TVS2 used in the snubber circuit unit 50 have different levels of inner capacitance because a semiconductor fabrication does not allow a complete uniformity of the products. Due to the different levels of capacitance, the voltages supplied to the respective first and second TVS diodes TVS1 and TVS2 are unequal.
FIG. 2 shows waveforms of the voltages supplied to the first and second TVS diodes TVS1 and TVS2.
Referring to FIG. 2, a waveform A represents total voltages that are supplied to both ends of the first and second TVS diodes TVS1 and TVS2 serially connected to each other, with the maximum value of 259V. A waveform Ref1 represents a voltage that is supplied to both ends of the second TVS diode TVS2, with the maximum value of 213V. A waveform Ref2 represents a voltage that is supplied to the first TVS diode TVS1, with the maximum value of 48.5.
As shown in FIG. 2, there is a difference between levels of the voltages that are supplied to the first TVS diode TVS1 and the second TVS diode TVS2. This difference is caused by the different inner capacitances as described above.
When the voltages are not equally supplied to the first and second TVS diodes TVS1 and TVS2 serially connected to each other, one of the first and second TVS diodes TVS1 and TVS2 may be supplied with a transient voltage exceeding a rated voltage. Such a transient voltage exceeding the rated voltage causes a burnt phenomenon such that the TVS diode is damaged and has its lifespan reduced. Accordingly, abnormality of the snubber circuit unit 50 is incurred and thus the total power supply circuit cannot be protected from the transient voltage.
The present invention has been developed in order to solve the above problems in the related art. Accordingly, an object of the present invention is to provide a power supply circuit for a video display device capable of performing a stable operation by improving transient state characteristics of respective TVS diodes used in a snubber circuit unit.
The foregoing object is realized by providing a power supply circuit for a video display device comprising: a power transformer for inducing a voltage with respect to an input voltage by using an interaction occurring between a primary coil and a secondary coil; a switching circuit unit for controlling the voltage to be induced at the secondary coil of the power transformer by switching on/off a current flowing along the primary coil of the power transformer; first and second TVS diodes serially connected to each other, for absorbing a transient voltage in a reverse direction that is supplied through the primary coil of the power transformer; first and second resistances parallel connected to the respective first and second TVS diodes; a capacitor parallel connected to both ends of the first and second TVS diodes connected to each other and being charged with the transient voltage in the reverse direction that is supplied through the primary coil of the power transformer; and a diode for forming a passage of current in one direction when the capacitor is charged. It is preferred that the first and second resistances have the same resistance values.
Preferably, the power supply circuit further comprises a rectifier circuit unit for rectifying an input AC power and supplying the rectified power to the primary coil of the power transformer, and an output circuit unit for rectifying and smoothing the voltage induced at the secondary coil of the power transformer.
It is preferred that the switching circuit unit comprises: a field effect transistor for switching on/off the current flowing along the primary coil of the power transformer; and a controller for controlling an on/off switching operation of the field effect transistor.
Also, it is preferred that a bead-core is connected between the field effect transistor and the primary coil of the power transformer, for removing noise.